The present invention relates to bipolar plates between adjacent electrochemical cells. More particularly, the invention relates to lightweight bipolar plates and methods for their construction.
Most of the components currently used in proton exchange membrane (PEM) fuel cells are derived from designs originally developed for use in phosphoric acid fuel cells (PAFC), and are not optimal for the higher performance of PEM fuel cells.
By the mid-70s, components consisting entirely of carbon were made for use in PAFC""s operating at temperatures in the 165-185xc2x0 C. range. One particular manufacturer has made bipolar plates by molding a mixture of graphite powder (approximately 67 wt %) and phenolic resin (approximately 33 wt %) and carefully heat-treating to carbonize the resin without introducing excessive porosity by rapid degassing. Typically, heat treatment to 900xc2x0 C. was sufficient to give the required chemical, physical and mechanical properties. The bipolar plates were molded flat and were machined to produce the required fluid distribution or collection grooves (or cooling grooves for the bipolar plate). Somewhat later in time, grooved plates were molded in a die (which was slightly oversized to compensate for shrinkage during baking) to produce a glassy graphitic, carbon-composite plate. However, while carbon/graphite bipolar plates are effective, they are expensive and, because it is difficult to produce thin carbon based bipolar plates, stack built with these plates tend to be heavy and bulky.
One alternative for overcoming these limitations is to use a moldable graphite-based composite that does not have to be carbonized. Graphite powder, which serves as the conductor, is bonded into a rigid piece with a polymer matrix. The graphite retains its conductivity and corrosion resistance, and the polymer binder, which must also be stable under PEM operating conditions, allows the plate to be formed by conventional polymer forming processes. This approach has distinct limitations. When the graphite is diluted with the polymer, its conductivity, already lower than any metal, is reduced even further. A seven kilowatt stack with pure graphite bipolar plates would be expected to have a 16 Watt internal resistive loss. When the graphite is dispersed in a polymer matrix, this loss will be larger.
Yet another example of a bipolar plate is a solid titanium metal sheet. The titanium is resistant to corrosion in many applications, provides greater electronic conductivity than does graphite, and can be made in relatively thin sheets. However, titanium is very expensive and relatively heavy itself.
Therefore, there is a need for a lightweight bipolar plate that provides the desired conductivity and can withstand the corrosive environments typically found in fuel cells and the like.
The present invention relates to bipolar separators or plates positioned between adjacent electrochemical cells, including lightweight bipolar plates and methods for their construction. The invention involves methods for depositing metals onto Mg and Al plates from either an aqueous solution or a non-aqueous solution of molten salts.
In one embodiment of the invention, Pdxe2x80x94Ni alloys can be electroplated from an aqueous solution onto electroless Ni deposits using direct current (DC), pulsed current (PC), and pulse reversal (PR) plating conditions. Using simple PC plating conditions, the size of electrodeposited Ni crystals were, significantly smaller than those obtained using DC plating, thereby, reducing porosity and improving the hardness of the coating. Pulse reversal plating yielded a Ni coating with improved corrosion resistance in various environments, because the most active metal crystals are continuously dissolved during the anodic pulse.
Because of the hydrophilic nature of gas flow channels in metal bipolar plates, the channels may flood with water and hinder the supply of gaseous reactants to the electrodes. Therefore, the surfaces of gas flow channels are preferably hydrophobic to prevent the attachment of water droplets. In order to make the channels hydrophobic, an electroplated metal-PTFE composite coating can be deposited on the surface of the Pdxe2x80x94Ni coating. The unique water repellent property of PTFE particles incorporated into the composite coating will prevent the penetration of liquid water into any pores in the deposited layer which will also aid in preventing corrosion of the metal substrate. The electroplated metal used in the composite coating can be selected from Ru, Pd, and Au.
In a second embodiment of the invention, a non-aqueous plating system, such as a molten salt solution, is used to plate metal onto the substrate. One difficulty associated with an aqueous system for plating an Al or Mg substrate is that the Al and Mg react with water to form an oxide film. Thus, the use of an aqueous plating system requires a multi-step, multi-layer plating process to remove the oxide film and plate the desired metals onto the substrate. By contrast, a non-aqueous solvent that does not react with Al and Mg substrates can be used to plate the desired plating metals directly onto the Mg or Al substrate.
Aluminum chloride (AlCl3) is one example of a covalently bonded compound useful as one component of a molten salt solvent. AlCl3 occurs as the dimer (Al2Cl4) and will readily combine with almost any free chloride to form a tetrahedral aluminum tetrachloride anion (AlCl4). This covalently bonded ion acts as a large monovalent ion, with the negative charge dispersed over a large volume. The complex salt (such as NaAlCl4) has a far lower melting point than the corresponding simple chlorides. The alkali metal tetrachloroaluminate complex salts (NaAlCl4 and KAlCl4) have been used as moderately high temperature solvents (for example at 150-300xc2x0 C.) for a variety of purposes, including electrochemistry, spectroscopy, and crystal growth.
Ambient temperature molten salts can also be formed from aluminum chloride. Table 1 lists some of the compounds capable of forming ambient temperature molten salts when combined with aluminum chloride. All of the materials listed are ionic chlorides. With the exception of TMPAC, all have the positive charge delocalized to some degree through a xcfx80-conjugated system over a large portion of the volume of the bulky cation. In all cases, the combination of a large cation, with a low charge density and a large anion, with a low charge density, leads to a low melting solid. The combination is an ionic liquid that actually behaves in some respects more like a molecular liquid. Unlike high temperature molten salts, which tend to interact only through non-directional charge-charge interactions, these molten salts are hydrogen bonded liquids with the cations forming a water-like network.
Transition metals are more easily plated from molten salts than reactive metals, since they are more easily reduced than the solvent, instead of being part of the solvent. In molten salts, like in aqueous solutions, the most easily reduced species will be reduced and deposited (plated) first, with that species protecting the solvent from reduction until it is consumed. Some of the materials plated, together with the base used in the solvent system used for plating are listed in Table 2.
Some of the elements in Table 2 were plated from acidic melts, others from basic melts. A few have been plated from both acidic and basic melts. The substrates used in these plating tests varied widely as well, with relatively refractory materials such as Pt and glassy carbon common, as well as Al.
Since Al and Mg are generally more electroactive than the metals to be deposited, it is likely that the material initially deposited will be an alloy of Al or Mg and the metal being deposited. As seen in Table 2, these types of alloys have been observed for several elements (Co, Ni, and Cu). In those cases, the careful and continued deposition of the transition metal leads to a pure transition metal surface. The elements were determined to behave in accordance with their position in the electromotive series and an examination of the appropriate binary phase diagram to identify potential for forming intermetallic phases.